1. Field of the Invention
The present invention relates to a mobile communication system and particularly to a variable rate CDMA spread circuit in a mobile phone or a cellular phone system (cellular system) using the direct spread code division multiple access (DS-CDMA) scheme, and more particularly, to a variable rate CDMA spread circuit in the transmission unit of a base station of a mobile communication system such as a multimedia service having a plurality of bit rate channels.
2. Description of the Prior Art
In conventional mobile communication systems, the North American standard system (TIA IS95) is known as a digital mobile phone or a cellular phone system (cellular system) using the code division multiple access (CDMA) scheme. The standard specifications TIA/EIA/IS-95-A published by TIA (Telecommunication Industry Association) describes in Chapter 6 an operation required of a mobile station, and describes in Chapter 7 thereof an operation required of a base station.
In a forward link (down channel: from base station to mobile station) based on IS-95, one of four bit rates, 9.6 kbps, 4.8 kbps, 2.4 kbps, and 1.2 kbps, can be selected to transmit/receive data. The basic rate is 9.6 kbps. When data is to be transmitted at a bit rate corresponding to 1/2, 1/4, or 1/8 the basic rate, data is repeatedly transmitted two, four or eight times to realize a variable rate transfer. When data is to be repeatedly transmitted two, four or eight times, the transmission power of each transmission is set to be 1/2, 1/4, or 1/8 of the basic power to respectively obtain constant transmission power per information bit. For the spread codes of the prior art, spread codes corresponding to the basic rate are used regardless of the bit rate being transmitted for the channel. More specifically, only 64 spread codes, limited by the spread rate of the basic rate, can be used per carriers independently of the bit rate.
In methods of transmitting/receiving a plurality of channels of different bit rates, methods disclosed in Japanese Unexamined Patent Publication Nos. HEI 7-177569 and HEI 6-237214 are known in which code division multiple access (CDMA) and time division multiple access (TDMA) are combined to change the number of time slots to be used in accordance with the bit rate. However, in this method using a combination of CDMA and TDMA, the range of bit rates which can be easily realized is limited by the number of time slots. When the number of time slots to be used is reduced to perform intermittent transmission, the transmission power control interval which is essential for CDMA becomes long and increases the transmission power control error. In addition, the interleave effect decreases. Furthermore, since the transmission/reception timings must be matched for all channels, the flexibility is poor.
In a system in which the voice service is mainly provided and the bit rate is lowered only in a silent section, as in IS-95, the prior arts pose no problem. However, a problem occurs in a multimedia service, where signals such as voice data, image data, and other data have bit rates for transmission which are different and simultaneously existent. If the spread codes of the basic rate are assigned to a channel which requires only a low rate, there is a problem that the spread codes are insufficient. For example, from the viewpoint of the usage of a radio band, a channel using only 1.2 kbps can accommodate, in one carrier, the eight-time channel of 9.6 kbps. However, only a two- or three-time channel can be assigned because sufficient spread codes cannot be assigned.
When a number of channels having bit rates lower than the basic rate are present in a CDMA system having a plurality of bit rates together, and the spread codes (short codes) for one basic rate are assigned to all the channels, the number of spread codes (i.e., number of channels) which are orthogonal to each other is few and the frequencies cannot be efficiently used.